Taxonomic and Functional Analyses of Marine Microbial Polysaccharide Utilisation

Abstract

Marine primary production accounts for half of the Earth' s carbon fixation and therefore has a significant impact on the global carbon cycle. However, although marine primary producers fix such a significant fraction of carbon, their biomass makes up only a small fraction of the total organic carbon pool. This is due to the extremely high turnover of phytoplankton and phytoplankton-derived organic matter within the oceans. This turnover is mediated by marine heterotrophic microorganisms (Bacteria and Archaea). Marine microorganisms therefore significantly affect the global carbon cycle. The objectives of this thesis were to investigate how the taxonomic and functional diversity of marine microorganisms affects the bacterially mediated carbon turnover in the Atlantic Ocean. In chapter 1 and 2 I developed methods which enable a shipboard high-throughput analysis of microbial diversity and abundance. These methods were developed to overcome the time delay between sampling and results and enable a comprehensive interpretation of the microbial community composition even in remote sampling sites. In chapter 3 I explored the biogeographical distribution patterns of the free-living (FL) and particle-associated (PA) bacterial communities across different provinces of the Atlantic Ocean. The FL and PA bacterial community compositions were more similar under copiotrophic condition and more dissimilar under oligotrophic conditions. I could associate these results to the relative age of the available particles as well as the availability of organic matter. In Chapter 4 I investigated alternative substrate uptake mechanisms in marine bacteria. Using fluorescently labelled polysaccharides (FLA-PS) and super-resolution structured illumination microscopy I could show that a significant fraction of marine bacteria use a a selfisha substrate utilisation mechanism. I combined this analysis with fluorescence in situ hybridization (FISH) to taxonomically identify the organisms as belonging to the Bacteroidetes, Planctomycetes and Gammaproteobacteria. The discovery of a widespread alternative substrate utilisation mechanism significantly affects our global estimates of carbon turnover by marine bacteria. Finally, in Chapter 5 I investigated the extracellular hydrolysis rates and bacterial community dynamics within fluorescently labelled polysaccharide incubations. These analyses lead to the identification of the dominant microorganisms associated with the hydrolysis of polysaccharides in the marine environment. The work done in this thesis has furthered our understanding of the activity and distribution patterns of bacterial polysaccharide utilisation mechanisms across the Atlantic Ocean. Additionally, the activity of the individual mechanisms could be associated with specific microbial groups, thereby linking the taxonomy and function of the dominant marine polysaccharide degrading organisms. This study will enable us to make better predictions of the impact which marine microorganisms have on global biogeochemical cycles.